Dissolved and colloidal substances (DCS) will be released during mechanical pulping and following bleaching operations. The accumulation of DCS in paper machine systems can bring various negative impacts on papermaking operations. In present study, the influence of alkaline treatment and alkaline peroxide bleaching of aspen chemithermomechanical pulp on released DCS was evaluated based on general DCS properties, lignin content, and carbohydrate and organic extractives compositions. Results showed that both treatments could promote the DCS release by increasing the concentration and cationic demand of DCS samples. However, alkaline peroxide bleaching caused the decrease in turbidity and average particle size. The total amounts of dissolved lignin, carbohydrates, and organic extractives were also increased. The dissolved lignin-related substances and carbohydrates were the predominant components of DCS. In the extractives, alkaline peroxide bleaching mainly resulted in the release of some lignin-degraded substances and a slight degradation of unsaturated fatty acids and their esters.